diff --git a/src/emqx_shared_sub.erl b/src/emqx_shared_sub.erl
index 675309480..c22e535c3 100644
--- a/src/emqx_shared_sub.erl
+++ b/src/emqx_shared_sub.erl
@@ -185,11 +185,15 @@ dispatch_with_ack(SubPid, Group, Topic, Msg, Type) ->
     Ref = erlang:monitor(process, SubPid),
     Sender = self(),
     SubPid ! {deliver, Topic, with_group_ack(Msg, Group, Type, Sender, Ref)},
-    OldRef = old_ref(Type, Group, Ref),
     Timeout = case Msg#message.qos of
                   ?QOS_1 -> timer:seconds(?SHARED_SUB_QOS1_DISPATCH_TIMEOUT_SECONDS);
                   ?QOS_2 -> infinity
               end,
+
+    %% This OldRef is a forward compatibilty workaround.
+    %% When an old version receives {Sender, OldRef} tuple, it acks (or nacks) back
+    %% with this OldRef structure
+    OldRef = old_ref(Type, Group, Ref),
     try
         receive
             {ReceivedRef, ?ACK} when ReceivedRef =:= Ref; ReceivedRef =:= OldRef ->
@@ -237,7 +241,7 @@ maybe_nack_dropped(Msg) ->
         ?NO_ACK                          -> false;
         {Sender, {fresh, _Group, Ref}}   -> ok == nack(Sender, Ref, dropped);
         {_Sender, {retry, _Group, _Ref}} -> maybe_ack(Msg), false;
-        %% This clause is for backward compatability
+        %% This clause is for backward compatibility
         Ack ->
             {Sender, Ref} = fetch_sender_ref(Ack),
             ok == nack(Sender, Ref, dropped)
@@ -268,7 +272,7 @@ maybe_ack(Msg) ->
     end.
 
 fetch_sender_ref({Sender, {_Type, _Group, Ref}}) -> {Sender, Ref};
-%% These clauses are for backward compatability
+%% These clauses are for backward compatibility
 fetch_sender_ref({Sender, {_Group, Ref}}) -> {Sender, Ref};
 fetch_sender_ref({Sender, Ref}) -> {Sender, Ref}.